Tacrolimus, also known as FK-506 or FR-900506, has the chemical tricyclic structure shown below:

corresponding to C44H69NO12. Tacrolimus appears in the form of white crystals or crystalline powder. It is practically insoluble in water, freely soluble in ethanol and very soluble in methanol and chloroform.
The preparation of tacrolimus is described in EP-A-0 184 162 and analogues of tacrolimus are disclosed e.g. in EP-A-0 444 659 and U.S. Pat. No. 6,387,918, which are both hereby incorporated by reference.
Tacrolimus is a macrolide compound with useful immunosuppressive activity, antimicrobial activity and other pharmacological activities and is of value for the treatment or prevention of rejection reactions by transplantation of organs or tissues, graft versus host diseases, autoimmune diseases and infectious diseases. Tacrolimus prolongs the survival of the host and transplanted graft in animal transplant models of liver, kidney, heart, bone marrow and small bowel and pancreas, lung and trachea, skin, cornea and limb.
In animals, tacrolimus has been demonstrated to suppress some humoral immunity and, to a greater extent, cell-mediated reactions such as allograft rejection, delayed type hypersensitivity, collagen-induced arthritis, experimental allergic encephalomyelitis and graft-versus-host disease.
Tacrolimus inhibits T-lymphocyte activation, although the exact mechanism of action is unknown. Experimental evidence suggest that tacrolimus binds to an intracellular protein, FKBP-12. A complex of tacrolimus-FKBP-12, calcium, calmodulin, and calcineurin is then formed and the phosphatase activity of calcineurin inhibited. This effect may prevent the dephosphorylation and translocation of nuclear factor of activated T-cells, a nuclear component thought to initiate gene transcription for the formation of lymphokines. The net result is the inhibition of T-lymphocyte activation, i.e. immunosupression.
Tacrolimus is extensively metabolized by the CYP3A4 isoenzyme in the gut wall and liver. Therefore, drugs that affect this isoenzyme may influence absorption and the subsequent elimination of systemically absorbed tacrolimus. Inhibitors of CYP3A4 may increase tacrolimus levels, while inducers of CYP3A4 may increase the metabolism of tacrolimus and decrease tacrolimus levels. Accordingly, tacrolimus may be administered together with one or more CYP3A4 inhibitors in order to improve the overall bioavailability.
Usually tacrolimus is administered orally and is therefore absorbed from the gastrointestinal tract. It has been observed that the absorption is negatively influenced by the simultaneous ingestion of food. Thus, the rate and extent of tacrolimus absorption were greatest under fasted conditions.
In general, it is known that the absorption and bioavailability of a therapeutically active substance can be affected by a variety of factors when administered orally. Such factors include the presence of food in the gastrointestinal tract and, in general, the gastric residence time of a drug substance is significantly longer in the presence of food than in the fasted state. If the bioavailability of a drug substance is affected beyond a certain point due to the presence of food in the gastrointestinal tract, the drug substance is said to exhibit a food effect. Food effects are important because absorption and hence the plasma levels becomes highly variable depending on food intake. Absorption into the bloodstream may be adversely affected to the point that the patient risks insufficient absorption to remedy the condition for which the drug was administered. On the other hand, the very high peak concentrations seen at fasted conditions occasionally, may very well induce significant side effects, of nephro- or neuro-toxic origin, as well as GI side-effects and others.
Absorption of tacrolimus from the gastrointestinal tract after oral administration is rapid with a mean time-to-peak concentration (tmax) of approximately 1-2 hours after administration to healthy subjects or kidney or liver transplanted patients, but incomplete and variable. The bioavailability is generally as low as at the most about 20% after oral administration.
Frequently observed side effects are vomiting and nausea but side effects like tremor, headache, hypertension, renal dysfunction, hyperkalemia, hypomagnesaemia, hyperglycemia, insomnia, diarrhea, constipation, abdominal pain, nephrotoxicity and neurotoxicity are also observed.
For oral administration, tacrolimus is currently formulated and marketed as soft gelatine capsules comprising the equivalent of 0.5, 1 or 5 mg anhydrous tacrolimus and marketed under the trade name Prograf® and Protropic®. The recommended initial oral dose is from about 0.1 to 0.2 mg/kg/day in patients. The dose aims at a certain trough plasma level from about 5 to about 20 ng/ml. Prograf® is indicated for the prophylaxis of organ rejection in patients receiving allogeneic liver or kidney transplants.
There remains a need for novel pharmaceutical compositions and/or dosage forms comprising tacrolimus exhibiting enhanced bioavailability. An increased bioavailability may allow a reduction in the dosage units taken by a patient, e.g. down to a single dose daily, and may also reduce or negate the need for food to be takes simultaneously with the dosage form thereby allowing patients more freedom on when the drug is taken. Furthermore, it is contemplated that fluctuations in the plasma concentration versus time profile may be significantly reduced. Further, enhanced bioavailability may also result in a more reproducible (i.e. less variable compared to that of Prograf®) release profile.